1. Field of the Invention
The present invention relates to servo devices which simultaneously control plural servo systems, using digital signal processors (DSPs) and similar calculation processing devices; and more particularly, devices which set the gains of servos, such that each servo is maintained unchanged.
2. Description of the Related Art
Servos (closed loop control systems), by applying, to a control subject, a control input comparing a target value and a control quantity from the control subject, perform control such that the target value and the control quantity are in agreement. The performance of servo systems is determined by how rapidly the control quantity reaches the target value (termed "rapidity of response" hereinbelow) and also, how stably (termed "stability" hereinbelow), the servo systems can control. In particular, what determines rapidity of response is the magnitude of the loop gain of the control system, and what determines stability is the phase margin and gain margin of the control system. From such parameters, generally through a sensor detecting the control quantity, and accurately computing the sensitivity of the control subject, the performance of the control system is increased, by determining the three parameters of loop gain, phase margin and gain margin.
However, in the control of an actuator used in an optical disc drive device, even if the gain is set accurately by determining the resonance of the actuator and the sensitivity of each unit, due to the differences in optical characteristics which accompany a change of discs, the loop gain which has been set changes. As a result, the phase margin and gain margin which determine the performance of servo system change, and the servo system has instability. Thus, the predetermined accuracy of controlling the actuator cannot be maintained.
On the other hand, in servo devices which simultaneously control plural servos using a digital signal processor (DSP) and similar calculation processing devices, the control of the plural servos causes the execution of calculation processing programs by which the gain of each servo has been set. In cases in which the gain is unsuitable, gain adjustment has to be performed by substituting a gain setting value, which has been changed in the program.
Nevertheless, when a servo device includes a DSP which has the minimum limiting performance necessary to operate plural servos simultaneously, and the process of measuring the servo gain afresh and resetting it is performed, the load of this process exceeds the processing capacity of the DSP. A problem occurs that the servo device can not set the gain. This problem may be solved if a DSP of high capacity is used, but such a high processing capacity DSP is very expensive.